As a method of processing the surface of a wafer or the like, plasma processing is carried out. As an apparatus for the plasma processing, there is used, e.g., a wafer-by-wafer processing apparatus in which a single wafer is loaded into and processed within a processing chamber every time a wafer is processed. In the wafer-by-wafer processing apparatus, in order to stabilize a gas concentration and a temperature within the processing chamber at the wafer processing startup time, a method is used in which a number of dummy wafers (distinguished from product wafers) for use in the production of semiconductor chips are continuously processed (preprocessing) and then the product wafers are processed (main processing). In some cases, if the product wafers are subjected to nitriding during the main processing without performing the preprocessing using the dummy wafers, the number of initially processed product wafers may have a low nitrogen concentration. Bare silicon substrates are generally used as the dummy wafers.
In order to reduce metal contamination when starting up an apparatus, plasma discharge is performed while supplying a nitrogen gas into a processing chamber when a wafer is not placed on a susceptor installed inside the processing chamber. The internal pressure of the processing chamber during the plasma discharge is kept lower than the pressure during the plasma processing of product wafers. If the internal pressure of the processing chamber during the plasma discharge is kept low, the impact forces of electrons or ions grow larger. Metallic contaminants adhering to those members arranged within the processing chamber are easily removed and discharged from the processing chamber.
In the conventional product substrate processing method, when a product substrate having an oxide film formed on the surface thereof is subjected to nitriding, there is a need to preliminarily nitride a multiple number of dummy substrates having an oxide film formed on the surface thereof.